Automatic telephone number identification apparatus is in widespread use in the United States and elsewhere for determining which individual telephone subscriber, or party, has placed a long-distance telephone call on a multiparty telephone line, in order that the party may be appropriately billed for the cost of the call. Such apparatus is also being increasingly used for determining the party that has placed a local telephone call. Typically, automatic telephone number identification apparatus is divided into two portions, the first being an apparatus which identifies which party on a line has made the long-distance call, and the second being an apparatus for forwarding to a toll or other central office the telephone number of the thus-identified calling party.
The present invention deals particularly with that portion of automatic telephone number identification apparatus which is used to identify the calling party. By far the most widespread identification method and apparatus known to the prior art is that commonly referred to as resistance ground automatic number identification which is particularly adaptable to those situations in which two parties share a common telephone line. Typically, one of the parties, known in the art as "party 238 , has located within each telephone instrument at its station a ground mark circuit, usually comprising the series connection of a resistance and an inductor which is connected to ground. For example, the ground mark circuit may comprise a portion of a coil for a ringer or bell in each telephone instrument. The telephone instruments of the other party, known in the art as "party 1", either do not have such a ground mark circuit or have the ground mark circuit therein disconnected. To provide detection of the calling party, a central office associated with the common telephone line applied a DC signal to the common telephone line upon detection of a long-distance call having been initiated on that line, usually shortly after the calling party has dialed a number for which a toll charge is to be made. If the long-distance call has been initiated by party 2, a DC imbalance between the tip (T) and ring (R) conductors of the telephone line resulting from the presence of the ground mark circuit is detected at the central office. If the call has been initiated by party 1, then no such DC imbalance is detected. The equipment at the central office then forwards the calling party's telephone number to a toll office for billing purposes.
Although simple in concept, construction, and operation, resistance ground automatic number identification presents significant problems to telephone companies in actual application. For example, each telephone instrument at each party station must have its ground mark circuit connected or disconnected in accordance with that party's designation as party 1 or party 2. It is sometimes inconvenient for a telephone company to connect or disconnect the ground mark circuit at the time of installation of each telephone instrument. Additionally, when a group of existing telephones are being converted from operator number identification to automatic number identification, burdensome problems of arranging for home visits to install ground mark circuits are presented. Furthermore, with the decline of telephone leasing, and with a corresponding increase in the number of telephones which are purchased and installed by subscribers, the control of a telephone company over the individual telephones in its system has significantly decreased to a point where the telephone company cannot assure that the telephone instruments of any given subscriber either have or do not have the ground mark circuit connected in accordance with that subscriber's party identification. Not surprisingly, these problems of installation and control have caused numerous errors in proper identification of and billing of subscribers placing long-distance telephone calls. Finally, the ground mark circuit itself and its connection to the telephone instrument must be carefully designed to minimize noise that may be present on the telephone line due to the ground connection in the ground mark circuit.
A solution to the aforementioned problems of installation, control and design of ground mark circuits is provided by the invention disclosed and claimed in copending U.S. patent application, Ser. No. 596,397, filed July 16, 1975, entitled "AUTOMATIC TELEPHONE NUMBER IDENTIFICATION CIRCUIT", by Darryl F. Proctor and Peter T. Skelly, now U.S. Pat. No. 4,001,512, Jan. 4, 1972 which is assigned to the assignee of the present invention. However, resistance ground number identification still cannot be used where more than two parties share a common telephone line.
Another two-party identification method and apparatus known to the prior art includes a pair of reverse-parallel diodes which are located at the party 2 station and in series circuit with the telephone instruments thereof and the portion of the telephone line "downstream" of the common connection of the party 1 and party 2 stations with that portion of the telephone line going to the central office. No such reverse-parallel diodes are provided in circuit with the instruments of the party 1 station. A controllable voltage source is connected to one side of the telephone line at the central office, and a voltage detector is connected to the other side of the telephone line at the central office. Upon detection of the placement of a long-distance call, the central office supplies a short across the telephone line to discharge any distributed capacitance therein. Shortly thereafter, a very small DC voltage pulse is applied by the controllable voltage source. If the telephone instrument going off-hook is in the party 1 station, this pulse will be reflected on the other side of the telephone line in the central office and will be detected by the voltage detector to signify that party 1 has placed a call. If the telephone instrument going off-hook is in the party 2 station, however, the voltage drop across the reverse-parallel connected diode pair will absorb the DC voltage pulse so that the voltage appearing on the other side of the telephone line at the central office has a value insufficient to trigger the voltage detector, therefore signifying that party 2 has placed a call.
Although avoiding many of the problems associated with resistance ground number identification, systems of this type encounter much difficulty in application in the situation where diode bridges used for polarity guards, bridge taps, line lifters and loop extenders have been installed in the line, all of which provide voltage drops which can absorb the DC voltage pulse. In addition, such systems are not applicable to party lines for more than two parties.
There are known to the prior art various method and apparatus for providing identification of a calling party for those party lines having more than two parties connected thereto. In one system utilizing such a method and apparatus, a circuit is connected to each telephone instrument in each subscriber station. Each circuit includes a diode and a resistor connected to ground, with the polarity of the diode connection and the resistance value of the resistor being unique for each subscriber station. In response to application to the line at the central office of a DC voltage having a predetermined polarity, a predetermined value and direction of current unique to the calling party exists on the line so that the calling party can be detected at the central office to provide party identification. Typically, a combination of polar and marginal relays is used to effect current polarity and current value detection. In systems of this type, only four parties can be detected, the diodes cause noise on the line, and all the other problems and limitations of the resistance ground number identification systems are encountered.
In yet another multiple party identification method and apparatus, a network is placed in circuit with each telephone instrument at the party 2, party 3, and so forth stations, with no such network being provided at the party 1 station. All the networks for each station are designed to conduct current upon the application of a predetermined voltage value thereto, usually with reference to ground, with the predetermined voltage values differing among the stations. For example, the network may include a neon tube in series-parallel connection with an adjustable resistor, with the resistor establishing a network breakdown voltage, or, the network may include a PNPN voltage breakdown diode, or its electrical equivalent including a zener diode and a switching network responsive thereto, with selection of the breakdown voltages of the neon tubes, PNPN diodes or the zener diodes being chosen to correspond to the aforementioned predetermined voltage values. A step voltage source is provided at the central office for applying, in sequence, increasing voltage values to the common telephone line. A detector is also provided at the central office for detecting when, in the sequence of voltage application, current flows thorugh the common telephone line in order to provide party identification. Systems utilizing this method and apparatus are disadvantageous, however, in that again, each telephone instrument must include a proper network, and in that a constant reference or ground potential must be provided at all of the instruments at all of the stations.
Still another approach in the prior art to multiple party identification is the provision of separate conductors for each telephone instrument interconnected with contacts which are actuated upon that telephone instrument going off-hook, with the conductors being brought back to the central office or to some other detection point. In this situation, the number of conductors required for multiple parties make such systems impractical for any widespread application.
Finally, the prior art also teaches a method and apparatus in which each telephone instrument is equipped with a tone generator which is enabled upon that telephone instrument going off-hook to apply a tone signal to the telephone line. If the tone generators are designed so that each generator emits a tone distinctive of the subscriber station at which it is located, then a receiver at the central office can provide party detection in response to the actual tone that appears on the line at the central office. As with the resistance ground and other methods and apparatus described above, each telephone instrument must be modified to include a proper identification circuit, or, tone generator. Further, the existance of tones upon the lines is likely to interfere, at some point in the telephone system, with conventional apparatus for detecting tones corresponding to the number that has been dialed and to other tones utilized in the system for interconnection purposes. The tones on the line are also quite audible to the calling party.
It is therefore an object of this invention to provide, for use as part of and in conjunction with an automatic telephone number identification apparatus, a method and apparatus for automatically identifying an individual calling party on a multiparty telephone line, which method and apparatus avoids the disadvantages of the prior art previously referred to.
It is another object of this invention to provide such a method and apparatus which can be used with substantially all multiparty telephone lines, including those telephone lines which have more than two parties connected thereto.
It is still another object of this invention to provide such a method and apparatus which do not require each telephone instrument at each party station to be modified, but only require that a modification be made to that portion of the telephone line extending between the telephone instruments at a subscriber station and a common junction of all the subscriber stations with the portion of the telephone line going to the central office, thereby allowing the modification to be made without access to the telephone instruments and without access to the premises in which the telephone instruments are located.
It is a further object of this invention to provide such a method and apparatus which does not require an earth ground reference at any point, thereby avoiding the problem of noise injection into the telephone line.
It is yet a further object of this invention to provide such a method and apparatus which will furnish reliable party identification even though diode bridges, bridge taps, line lifters or most types of loop extenders are installed in conjunction with the telephone line.